Methods of fabricating a silicon oxide layer using an inorganic silicon precursor and methods of fabricating a semiconductor device using the same are provided. The methods of fabricating a semiconductor device include forming a tunnel insulating layer and a charge storage layer on a substrate; forming a dielectric layer structure on the charge storage layer using an atomic layer deposition (ALD) method, the dielectric layer structure including a first dielectric layer formed of silicon oxide, a second dielectric layer on the first dielectric layer formed of a material different from the material forming the first dielectric layer, and a third dielectric layer formed of the silicon oxide on the second dielectric layer; and forming a control gate on the dielectric layer structure. The first and third dielectric layers formed of the silicon oxide are formed using a first gas including an inorganic silicon precursor, a second gas including hydrogen gas or a hydrogen component, and a third gas including an oxide gas.